Vyvyan Basterd
Adventurer
I had this overpopulated class problem at the end of 3e. How do I include the myriad of classes into a believable fantasy world?
The trick to world-building is not to build too much detail in that you trap yourself.
* Instead of a Wizards Guild you create an Arcane Academy (which can teach/support all arcane classes).
* Recognize that groups in your campign world should not define what a character is. It is perfectly all right for a preist of a deity to be a class other than a Cleric and derive his powers from other than the divine power source.
* Be adaptable. New classes either always existed or a new way of doing things has developed.
* The only part of the world the characters should really know if the part that touches them. There should be a good feel for the general tone of the campaign world, but you don't need to create the entirety of the world that is "off-screen."
* DMs have never had a problem adding new monsters into their campaigns but I have seen many struggle with including new classes. Think of what justifies adding in that cool new monster and apply that understanding to other new aspects of the game.
* Remember that the ultimate goal is to have fun. You may have fun detailing your world beyond any scope that the players will ever see. Just think of new classes as another chance to create a part of your game world.